Blinds in poker are the obligatory ‘bets’ (in a game such as Hold’em) made even before any cards have been dealt (hence the name), by the two players to the immediate left of the Dealer, the idea being to generate action by already having chips/money in the pot. The first position is known as the Small Blind (e.g. 10 chips), and the next the Big Blind (e.g. 20 chips).
The action starts with the player to the left of the Big Blind (BB), the options being to Fold, Call (with a bet matching the value of the Big Blind) or Raise, continuing around the table to the Big Blind. If there has been no raise the Big Blind has the option to do so. The Dealer button continues clockwise around the table so that every player pays out the Blinds during an orbit of hands.
Note that in a Cash game the blinds remain constant, with the BB usually a hundredth of the maximum table limit (e.g. a BB of $2 in a $200 Max – or $1/$2 – game). In tournaments, on the other hand, the Blinds increase periodically after pre-set intervals known as Blind Levels. These can be anything from one to 20+ minutes depending on the format.